Results are reported of an experimental investigation of fully developed forced convection between two horizontal plates when the lower plate is heated and the upper one is cooled. The experiments covered a range of Rayleigh numbers between 100 and 13,500 and Reynolds numbers less than 100 using air. The heat transfer measurements show a sharp increase of heat transfer coefficient when longitudinal vortex rolls appear in the passage which indicates the considerable heat-transfer augmentation obtained in this way. Comparison of heat transfer coefficients for fully developed cases and those for confined cases indicates that there is no appreciable difference between them. Periodic spanwise temperature distributions created by vortex rolls are distorted by a second type of vortex rolls as the Rayleigh number increases. Because of the interaction the regular vortex structure disappears around Rayleigh number 8000.